Methods for making half-tone prints

ABSTRACT

THIS INVENTION RELATES TO A METHOD FOR THE PRODUCTION OF A PRINTING PLATE CAPABLE OF REPRODUCING CONTINUOUS TONES, WHEREIN A LIGHT-SENSITIVE MATERIAL SUITABLE FOR THE PRODUCTION OF A PRINTING PLATE IS EXPOSED UNDER A CONTINUOUS TONE ORGINIAL, EXPOSURE LIGHT BEING CAUSED TO TRAVERSE A LAYER WHICH PRESENTS SUFFICIENTLY UNIFORMLY DISTRIBUTED AREAS OF 0.05 TO 10 MICRONS IN DIAMETER, THE LIGHT:TRANSMISSION CAPACITY OF WHICH AREAS CONTRASTS WITH THAT OF THEIR IMMEDIATE ENVIROMENT, THIS CONTRAST LAYER BEING OF THE NATURE OF A SO-CALLED SHADOW PATTERN, AND THE EXPOSED MATERIAL IS DEVELOPED AND, IF DESIRED, ETCHED.

United" States Patent 3,829,315 METHODS FOR MAKING HALF-TONE PRINTSGunther Schadlich, Wiesbaden-Biebrich, Renate Haenisch, Wiesbaden, andRoland Moraw, Wiesbaden-Biebrich, Germany, assignors to KalleAktiengesellschaft, Wiesbaden-Biebrich, Germany No Drawing. ApplicationSept. 8, 1971, Ser. No. 178,828, which is a continuation of applicationSer. No. 732,024, May 27, 1968, now abandoned. Divided and thisapplication Oct. 20, 1972, Ser. No. 299,185

Int. Cl. G031? 7/02, 7/24, /00

US. Cl. 9633 4 Claims ABSTRACT OF THE DISCLOSURE This is a division ofapplication Ser. No. 178,828, filed Sept. '8, 1971, which is, in turn, acontinuation of Ser. No. 732,024, filed May 27, 1968, now abandoned.

This invention relates to improvements in and relating to methods formaking continuous tone prints.

it is known that, for producing lithographic or intaglio printingplates, use can be made of a material having a light-sensitive copyingcoating, and that such material is exposed under an original to bereproduced, is then developed and, if necessary, etched.

It is also known, for the production of a printing plate reproducingcontinuous tones, to cause a finely divided shadow pattern, togetherwith the exposure pattern, to fall upon the copying layer duringexposure, by causing the exposure light to pass through a pattern offine, closely juxtaposed, crossed lines which do not transmit light orreduce the strength of the light, this pattern being known as a screen.Instead of line screens, dot screens can also be used; in these, dotswhich do not transmit light or which reduce its strength are disposed ina uniform lattice arrangement.

There must be a certain minimum distance between the lines or dotscomprising the screens. Many of the fine details of a continuous tonepicture therefore can be reproduced only incompletely. Thus, forcontinuous tone reproductions of lower quality, screens having 30 linesor dots per cm. of length are used, i.e., the so-called 30 screen; forfiner prints 60 screens are employed and in exceptional cases 120screens. The latter can be produced only by experienced and skilledtechnicians. Screens having a still greater degree of fineness cannot beproduced in a manner suitable for practical use. A further disadvantageof many screened continuous tone printing plates is the fact that aso-called moir effect occurs which affects reproduction of the picturein a manner that is usually not desired. A further disadvantage is thatproduction of the necessary screen films is relatively expensive.

It is also known to print continuous tone pictures by the offset processfrom aluminum plates with fine-grained surfaces, without the use of ascreen pattern. In this case printing is done from the grain of theplate. The printing process is known as screen-less offset printing.This process is free from the three disadvantages men- 3,829,315Patented Aug. 13, 1974 tioned above, but another disadvantage, causinggreat difiiculties, is associated therewith, in that it enables onlyrelatively few shade gradations to be reproduced. Also, no procedure isyet known for obtaining printing plates in a reliable manner.Furthermore, the number of prints obtainable from a plate is relativelysmall.

The present invention provides a method for the production of a printingplate capable of reproducing continuous tones, wherein a light-sensitivematerial suitable for the production of a printing plate is exposedunder a continuous tone original, the exposure light being caused totraverse a layer which presents sufliciently uniformly distributed areasof 0.05 to 10 microns in diameter of which the light-transmissioncapacity contrasts with that of their immediate environment, thiscontrast layer being of the nature of a so-called shadow pattern, andthe exposed material is developed and, if desired, etched.

This method of printing does not involve the drawbacks of screenprinting, but permits, in a reliable manner, the reproduction of agreater contrast range than the simple screen-less offset printingprocess and, furthermore, also can be used in lithographic and intaglioprinting.

The method according to the present invention, as in the known screenprocesses, employ-s exposure by means of a light-transmitting diaphragmwhich produces a finely distributed shadow pattern. In the methodaccording to the invention, however, the finely distributed shadowpattern produced is formed of smaller areas than in the case of thescreen process, i.e. of areas 0.05 to 10 microns in diameter, whereas,with the known dot screen, the diameter of the dots is well in excess of10 microns. The areas providing the shadow pattern in the case of theknown screens are of equal size to one another, whereas in the method ofthe invention they may be of different sizes. Furthermore, in the methodaccording to the invention, in contrast to the regular periodicrepetition of the arrangement of known screen patterns, the areasproducing the shadow pattern are preferably distributed in a randommanner. This affords the advantage that it is very much easier in mostcases to produce the areas providing the shadow pattern. The use ofmaterials in which the areas producing the shadow pattern are arrangedwith periodically repeated regularity, for example at the same distanceapart, is included in the method of the invention as long as the areashave a diameter of 0.05 to 10 microns.

The light-transmission capacity of the areas producing the shadowpattern contrasts with that of their immediate environment. The areasare thus capable of transmitting either less light or more light than istheir immediate environment. In the first case, the areas, and in thesecond case, the immediate environment of the areas, can be completelyimpenetrable by light. The random distribution of the areas of coursealso includes the possibility of the individual areas overlapping oneanother. The size of the total area of greater light-transmittingcapacity formed by the small individual areas also can vary within widelimits in relation to the size of the total area or lowerlight-transmission capacity. The ratio between the two is, however,normally between 1:9 and 9:1. It is preferable if the total area ofgreater light-transmission capacity is less than or approximately equalto the total area of the places throwing the shadows. The preferredratio of light spots to shadow spots is between 4:6 and 5.5 :4.5.

The method according to the invention can be carried out in difierentways. Thus, in the method for producing a printing plate for reproducingcontinuous tones, during exposure, for producing the shadow pattern, afilm can be placed between the light source and the copying coating,which film is provided with the areas having a lighttransmittingcapacity which contrasts with their environment. The film can be laiddirectly on the copying coating or it can be laid over the copyingcoating and separated therefrom by the continuous tone original.Instead, a coating incorporating the areas producing the shadow patterncan be carried directly on top of the copying coating. Alternatively,the copying coating itself can incorporate the areas producing theshadow patterns. If an objective, e.g. a projector, is used for theexposure, then the coating provided with the areas is arranged in theplane of the image or in its immediate vicinity.

The brighter the part of the image of the continuous tone original, usedduring exposure, the greater is the amount of light reaching thecorresponding part of the image on the printing plate during exposureand the stronger is the radiation penetrating beneath the intermediateshadow-projecting areas supplying the shadow pattern. This penetrationof the shadow-projecting areas, which increases with exposure time,brings about the continuous tone effect. It should be particularlystressed that the other good properties of the printing plates, such asdevelopability and abrasion resistance during printing are not adverselyaffected by the method of the invention.

There are many possible ways of producing the coating comprising theareas the light-transmission capacity of which contrasts with that oftheir environment. For example, areas of lesser light-transmittingcapacity and distributed in a random manner can be obtained by treatingtransparent films with a sandblast or by surface roughening, and thedepressions thereby formed can, in part, be filled with pigmentmaterials for enhancing the contrast in light-transmission capacity.Films of transparent materials can be cast or extruded with pigmentsubstances embedded therein. The desired result is also achieved by apartial vapor treatment of a transparent carrier or of the materialserving to produce the printing plates, using metals or other opticallymasking materials, or such materials can be deposited chemically orelectro-chemically through a mask containing perforations of appropriatenumber and size. Another method consists in spraying with colorantparticles or other optically dense substances. It is possible instead toburn holes, distributed in a dense random arrangement and of a fewmicrons in diameter, by means of focused electron beams, into atransparent film metallized or pigmented over its entire surface.Fabrics of appropriate fineness can be used as ultra-fine periodicallyrecurrent patterns. Exposure through sheets or bundles of glass fibersalso can lead to the desired continuous tone reproduction, since suchglass fibers can be produced from light-conducting nuclear fibers indiameters as low as microns. By using such light conductors it ispossible to bridge fairly large gaps between the original and theprinting plate. Metals deposited in finely particulate form embedded inbinding agents, for example photographic films exposed over their entiresurface and developed, or coatings of pigments in finely particulateform on transparent films or on the printing plate itself can be used asthe material for producing the shadow pattern.

Coatings of pigment or metal grains can be applied without complicatedtechnical means, so that their use for carrying the invention intoeffect is of considerable importance. Apart from the question of grainsize, the choice of pigments is practically unlimited. When usingcolorless pigments, e.g. powdered glass or glass beads, care must betaken to prevent fine dark portions of the image being at the same timeunder-penetrated due to over-long exposure, since these pigmentspreferentially scatter the irradiation light in a diffuse manner and donot absorb it. The necessary exposure times are quite short. Dark, oreven black pigments, such as fine carbon black, absorb over the entirevisible and adjacent ultraviolet spectrum range. A disadvantage here,however, is that, as a consequence of the intense absorption of light,the dark coating heats up considerably so that, upon exposure in thecopying frame, due to the good contact with the copying coating, thelatter becomes heated undesirably. It is therefore better to use thosepigments that absorb only in the spectrum sensitivity range of thecopying coating. In the case of copying coatings comprising aromaticdiazo compounds and azido compounds, as well as with many polymerizablephotographic coatings, it is therefore advantageous to employ yellow toyellow-brown pigments.

In the practical application of the method, the size of the areasproviding the shadows is very important. If pigments are used as thescreen substances, the most suitable grain sizes can he arrived atrelatively simply by sorting. It has been found that the mostadvantageous size range is between 0.5 tond 10 microns grain diameter.It is difiicult to determine whether there are useful extensions of thisrange in the directions of larger or smaller values, since there is acertain distribution of grain size in the coating. Good results havebeen obtained with a yellow pigment in respect of which themanufacturers quote a mean grain diameter of 0.48 micron. This pigmentwas dispersed in water, together with wetting agents and water-solublebinders, in a ball mill. The diameters of the largest particles in driedcoatings of this dispersion were assessed at about 5 microns whenobserved under the microscope. The pigment particles must, on the otherhand, be distinctly smaller than the screen dots of the much used 60screen, since otherwise shadow areas, as large as these screen dots, nolonger can be under-penetrated in reasonably short exposure times andthey cause correspondingly large flecks on the printing plate. Thepigments however should not be too fine, since otherwise they produce ahaze effect, which lengthens the exposure time. As tests with pigmentshaving the appropriate degrees of fineness have shown, the lowerpermissible limit for grain size is roughly 0.05 micron. Whether coarseror finer pigments are used, within the recited particle size range, willdepend upon the purpose for which the process is used. Pigments that aretoo coarse, however, cause pronounced structuring of the picture, sothat the use of pigments including particles of diameters greater than10 microns is best avoided.

The above considerations regarding the most favorable particle sizerange of the shadow-forming areas in pigment coatings also apply in thecase of other coatings, for example vapor-deposited metal coatings. Themethod for determining the optimum pigment density, to be describedlater, applies in an analogous manner to the production of othercoatings, for producing the shadow pattern. In general, it has beenobserved that the reproduction of continuous tones is better with apigment coating of up to a certain thickness, the necessary exposuretimes also becoming longer. Good results are no longer obtained withpigment coatings that are too thick. For a given pigment layerarrangement and a given pigment composition it is possible to determinethe optimum pigment density, for example, by increasing the pigmentdensity until, upon printing the image-wise exposed and developedprinting plate, the image of the original is repoduced with the correctillumination distribution. Under the conditions used in Example 5 below,the weight of coating amounts, for example, to about mg. for a sizegiven by German Standard Specification DIN A 4. The most advantageousthickness of pigment coating and/or the best type of pigment arediflicult to predict for an individual case. It can be taken, however,as a general indication that the ratio of the open to the covered areashould not be much greater than 1; preferably, the covered area shouldbe greater than the open area. The complete printing system, includingprinting plate, printing machine and paper, with correct adjustment,operates with a gradation of 1. Accordingly, by varying the density ofthe pigment, the printing system can be made to give harder or softerresults, and it is even possible, if re quired, to perform tone valuecorrections. The problem is usually to reduce the gradation of theprinting system by the above-described measures. It has been found thatthe exposure time is thereby increased to roughly three or four timesthe exposure time required when no pigment coating is present.

Coatings containing pigment must incorporate a binding agent, so thatthe pigment adheres and cannot be wiped off.

When the pigment coating is applied to the copying coating of thematerial for producing the printing plate, the binding agents used mustbe such that the pigment coating is removed when developing the exposedmaterial with the developer solution. The binding agents employedpreferably should be soluble only in acid or better still, only inalkaline aqueous solutions, so that the applied pigment coatings areresistant to handling. The binding agents and the pigment materialsmust, of course, be compatible with the light-sensitive coating and, ifnecessary, stabilizing agents may be added. In the case oflightsensitive coatings based on diazo compounds, these are generallyorganic acids. The binding agents must meet no other specialrequirements, so that there is a large selection available. Bindingagents that are soluble only in organic solvents can be used if thenecessary solvents do not attack the printing plate material coating orsupport.

A particular advantage of partly light-transmitting metallic coatings isthe fact that they do not become appreciably heated even when exposed tolight for a fairly long period, since they do not absorb the radiatedlight. In the same way as pigment coatings, metals in the form of finegrains embedded in binding agent can be applied to the material forproducing printing plates. Metals can be obtained in the form of finegrains by reducing corresponding metallic salts. Technically producedpigments usually must be dispersed in the binding agent solution bymeans of a ball mill, and isolated coarse particles of pigment findtheir way into the suspension. With coatings of metal grains produced inthe same way, however, particles are obtained the upper size limit ofwhich is better controlled. Prints from printing plates produced withsuch metal grain coatings therefore appear very smooth and attractive.Particularly uniform, fine-grained metal coatings are obtained byapplying the art of photographic film manufacture, by reacting silvernitrate with potassium bromide in a binding agent to produce silverbromide, followed by ripening and exposing and developing the washedemulsion. The required grain size can be obtained by the ripeningtreatment.

The continuous metallization of continuous strip material in a vacuum iswell known. Care must be taken in the application of this method that,during the vapor treatment, masking areas are formed of the sizereferred to in the case of pigment coatings. In this connection a finecluster of jets of metal can be confined and directed in pulses upon thematerial to be vapor-treated, or the metalvapor treatment can be appliedthrough a suitable screen. A still simpler method is by vapor-treatmentapplied obliquely to the normal to the surface. Printing plate supportssuch as aluminum are often roughened before application of thelight-sensitive coating, the rough surface structure being largelyretained through the applied lightsensitive coating. With the obliquevapor-treatment, metal is accordingly preferentially deposited on thepeaks. The same effect is achieved in the oblique vapor-treatment ofsuperficially roughened films. When a material for producing theprinting plates, and intended later to be developed with an aqueousdeveloper solution, is treated directly with metal vapor, it isadvantageous to apply a thin and very readily water-soluble coating tothe material so that, after exposure, the metallic coating can be moreeasily lifted by rinsing during the aqueous developing operation, andthus removed. The amount of metal to be vapor-deposited is determined bytests similar to those described above for determing the thickness ofpigments. Similar considerations apply as regards the density ofmetallic coatings which are deposited chemically or electrochemically.

The above-described procedures are suitable for producing positive andnegative offset printing plates. Relief and/or intaglio printing platesalso can be produced. In the etching operation however, fine dots on thescreen easily can be under-etched and their mechanical stability is low.In the case of offset printing plates, the method is independent of theprinting plate support. Use can be made of completely smooth supportssuch as multi-metal plates, polished aluminum, mechanically roughenedcarriers, carriers of anodized aluminum and also paper supports. Thebest results have been obtained with smooth or moderately roughenedsupports, of which the depth of the roughening is up to about 1 micron.As described in the Examples below, up to eight steps of the Kodak(Registered Trade Mark) No. 2 Photographic Step Wedge can be reproducedin the printing, this corresponding to the tone-graded reproduction of anominal density of 1.2. However, this certainly does not mean that thisis the limit of the performance of the method.

To obtain satisfactory results, the correct exposure time must beselected in a more precise manner than in the case of methods employinga screened original, if bright as well as dark toning is to bereproduced with gradation. Also, the correct setting of the printingsystem as regards the supply of damping Water and ink contributes toobtaining optimum results by the continuous tone reproduction methodaccording to the method of the invention. High quality paper with sealedsurfaces is also particularly suitable.

The following Examples further illustrate the invention:

Example 1 Commercially available materials for producing offset printingplates, provided on a brush-roughened aluminum support of about 2 to 3microns roughened depth with a positive working copying coating, inwhich is contained, as the light-sensitive substance a naphthoquinonediazide sulfonic acid ester, are coated in a centrifuge with aqueousdispersions in which carbon black, active carbon or other black pigmentsare dispersed with the sodium salt of diisobutylnaphthalene sulfonicacid being added as a Wetting agent. Dispersions of benzene of blackprinting ink and electrophotographic developer (toner) were also usedfor the coating. Viewed under the microscope, the pigment particles onthe dried coatings are between 5 and 10 microns in size. In accordanceWith the directions contained in the above general description, thedensity of the pigment particles is kept at a level such that thecontinuous tones are reproduced in the optimum manner. After exposureunder an arc lamp and a Kodak Photographic Step Wedge No. 2 with adensity increment of 0.15 per step, and after developing and inking upwith greasy, black printing ink, several tone-graded steps more areobserved than in the commercial material without the pigment coating.The exposure time for the pigmented printing plate material is longer,however. The plates coated with the aqueous dispersions are not fast towiping, but the pigment coating must be washed otf with benzene beforedevelopment with the alkaline developer can be carried out.

Example 2 A light-sensitive material of the kind used in Example 1 iscoated with a preserving lacquer which is an aqueous solution ofcellulose ether and gum arabic, and the layer in the tacky condition isdusted with a pigment. The dry printing plate is fast to wiping. Thefurther treatment and the results obtained are similar to thosedescribed in Example 1. The pigment used was carbon black having aparticle size range of 2 to 10 microns.

Example 3 A light-sensitive material of the kind used in Example 1 isprovided with a pigment coating of the following composition: 20 g. ofpoly-N-vinylpyrrolidone and 2 g. of diisobutylnaphthalene sulfonic acidsodium salt are dissolved in 300 ml. of warm water; 12 ml. of blackIndia ink are added to this solution. The suspension is put through acoarse filter and the filtrate is poured onto the rotating printingplate material (160 r.p.m.). The air movement produced by the rotationpromotes drying. The pigment particles in the dried coating are lessthan 1 mlCI'OIl.

The material coated in this way is exposed for 15 minutes under an arclamp and under a Kodak Photographic Step Wedge No. 2 with a densityincrement of 0.15 per step. After this time, full exposure of thematerial is just completed under the first step. The pigment coating iswashed off with Water and the material is developed by immersion for 1minute in an aqueous alkaline developer. After inking up with greasy,black printing ink, nine to ten tone-graded steps can be seen on theprinting plate, of which about eight are reproduced upon printing. Theextraordinarily long exposure time is attributable to the very fineparticles employed.

A printing plate material without a pigment coating, but otherwisetreated in the same way, requires an exposure time of about 1 minute andshows up to four steps, of which about three are reproduced uponprinting.

Example 4 A light-sensitive material of the kind used in Example 1 isprovided with a pigment coating of the following composition: 20 g. ofpoly-N-vinylpyrrolidone, 2 g. of diisobutylnaphthalene sulfonic acidsodium salt and 9 g. of Fixoplast Yellow S 501 opaque (a yellow pigmentsupplied by Farbenfabriken Bayer AG., Leverkusen, Germany) are dissolvedor suspended in 300 m1. of warm water and the product allowed to standfor a few minutes, whereby the coarse particles settle out. Thesupernatant liquid is poured on the rotating printing plate material(160 r.p.m.). The air movement occurring during rotation promotesdrying. After drying, a coating is obtained which comprises particleshaving an estimated diameter of 1 to microns and a surface covering ofabout 50%.

The printing plate material coated in this way is exposed for 3 minutesunder an arc lamp and under a Kodak Photographic Step Wedge No. 2 with adensity increment of 0.15 per step. After this period, the printingplate material is just fully exposed under the first step. The pigmentcoating is washed off with water and the printing plate material isdeveloped by immersing for 1 minute in an aqueous alkaline developer.After inking up with greasy, black printing ink, nine tone-graded stepscan be seen, of which about seven are reproduced upon printing.Continuous tone prints of good tone value gradation and resolutioncapacity are reproduced.

A printing plate having no pigment coating, but otherwise treated in thesame way, requires an exposure time of about 1 minute and shows aboutfour steps of which about three are reproduced upon printing.

Example 5 A light-sensitive material of the kind used in Example 1 isprovided with a pigment coating as follows:

1 ml. of Arcopal N 100, a wetting agent, marketed by Farbwerke HoechstAG., Germany, based on nonyl phenol, oxidized with a 10-fold molaramount of ethylene oxide, is dissolved in 80 ml. of warm water, andwhile stirring vigorously, 2.5 g. of Hansa Yellow 10 G (FarbwerkeHoechst AG.) are added this being a yellow monoazo pigment (Color IndexNo. 11,710) having a grain diameter of about 0.48 micron. After completedispersion, a binding agent suspension, that has been well mixed whilestirring, is added also with stirring. This suspension consists of 1 g.of poly-N-vinyl-N-methyl-acetamide 'K=88.1) in the form of a- 10%aqueous solution and 1 g. of polyvinyl acetate, also in the form of a10% aqueous solution. In all, the suspension thus amounts to 100 ml.This is ground for at least 3 hours in a ball mill until a uniformdistribution is obtained. The suspension is used immediately.

The printing plate material is coated by means of a doctor (0.1 mm. gap)and dried in air. It is possible instead to apply the coating using acentrifuge, but in that case the proportion of pigment must beapproximately doubled. After drying, a coating comprising particleshaving a mean diameter of about 5 microns is obtained.

The coated printing plate material is exposed for about 4 minutes underan arc lamp and under a Kodak Photographic Step Wedge No. 2 having adensity increment of 0.15 per step; the printing plate is then justfully exposed under the first step. After washing off the pigmentcoating with water, and then developing by immersion for 2 minutes in anaqueous alkaline developer and inking up with greasy, black printingink, nine to ten graded steps can be observed. Upon printing, seven toeight steps are reproduced. Continuous tone originals give reproductionshaving good gradation agreement and resolution capacity.

A printing plate material, having no pigment coating, but otherwisesimilarly treated, requires a period of about 1 minute under the firststep to give a corresponding full exposure. After developing and inkingup, about 3 to 4 steps can be seen, of which up to 3 are reproduced uponprinting.

The picture becomes somewhat smoother and the developing easier, if,prior to applying the pigment suspension, the printing plate material isgiven a preliminary coating consisting of a solution of a water-solublebinding agent, for example a 5% solution of poly-N-vinyl-N- methylacetamide.

Example 6 The same procedure is used as that described in Example 5, buta negative working offset printing plate material is employed, thecopying coating of which contains, as the light-sensitive substance, ap-iminoquinone diazide (in accordance with German Patent SpecificationNo. 1,104,824). The printing plate material, provided with the coatingof Hansa Yellow pigment, requires an exposure time of 45 sec. Afterdeveloping, with aqueous alkaline developer and after inking up, sixgradation steps can be seen.

A material which is the same except that it has no pigment coating,requires 15 sec. for the exposure and shows only about 1.5 steps.

Example 7 The same procedure is used as for Example 5, but a positiveworking offset printing plate material is used, of which the aluminumsupport is not mechanically roughened, but is oxidized by electrolytictreatment.

The printing plate material, provided with the coating of Hausa Yellowpigment, requires an exposure time of 6 min, After development with theaqueous alkaline developer and after inking up, four or five gradationsteps can be seen.

A material that is the same except that it has no pigment coating,requires a development time of 1 min. and shows only about two steps.

Example '8 A polyethylene terephthalate film, rendered matte by abrasionto give a roughened depth of up to 5 microns, is provided with a pigmentlayer of Hausa Yellow 10 G by the procedure given in Example 5. Thefilm, together with the pigment coating, is applied to the copyingcoating of a light-sensitive material of the kind used in Example 1, aKodak Photographic Step Wedge No. 2 is laid thereon, and exposurecarried out in a vacuum frame under an arc lamp. With the film applied,it is necessary to expose roughly four times longer than when exposingwithout application of the film. On the printing plate exposed under thepigment coating, then developed and inked up, two extra steps can beseen in comparison with a printing plate treated in exactly the same wayexcept that it was not exposed under a pigment coating.

Similar results are obtained when a clear cellulose acetate film is usedinstead of the matte polyester film.

Example 9 The same procedure as in Example is followed, but, as theyellow pigment, there are added 3 g. of Permanent Yellow 66 extra(Farbwerke Hoechst AG.), a yellow diazo pigment (Color Index No. 21,105)having a grain diameter of about 0.05 micron. On the dried printingplate material there is obtained a layer comprising particles of up to 1micron in diameter, but mostly finer.

After exposure for 12 minutes followed by developing and inking up,eight steps can be seen. A similar material, without a layer of pigment,requires an exposure time of 1 minute and reproduces up to four steps.

The extraordinarily long exposure time is attributed to the very smallsize of the particles of the pigment employed.

Example A commercial positive working offset printing plate material ofthe kind used in Example 1 is coated by the method of Example 5 with apigment suspension produced as follows: 6.25 g. of barium chloride and6.6 g. of aluminum sulfate (with 18 moles of water of crystallization)are dissolved separately, each in ml. of water. The two solutions arepoured together without stirring and left to stand for several hours.During this time barium sulfate in the form of very fine flockprecipitates and settles. The supernatant liquor is then decanted andreplaced by the same quantity of dis-tilled water. To this dispersion isadded 0.5 ml. of Arcopal N 100 and 50 ml. of a 10% aqueous solution ofpoly-N-vinyl-N-methylacetamide (K=88.1) is added as a binding agent. Themixture is stirred vigorously for 1 hour to achieve a uniformdistribution. The particles in the dried coating obtained with thisdispersion have a diameter of 1 to 5 microns.

After 2 minutes exposure under a Kodak Photographic Step Wedge No. 2having a density increment of 0.15 per step, the resulting offsetprinting plate material is developed with an aqueous alkaline solutionand inked up with greasy, black printing ink. Nine gradation steps canbe seen. The picture looks very smooth.

In the case of a similar plate without a pigment coating, an exposuretime of 1 minute is required. Three steps are reproduced.

Example 1 1 A commercial positive working offset printing plate materialof the type used in Example 1 is coated, in the manner described inExample 5, with a suspension consisting of 5 g. of finely divided silica(mean grain size about 5 microns), 50 ml. of water, 1 ml. of Arcopal N100 and 50 ml. of a 10% aqueous solution of poly-N-vinyl-N-methyl-acetamide '(K=88.1) and produced by grinding theconstituents for 3 hours in a ball mill. The paticles in the coating are5 to 10 microns in size.

The printing plate material thus coated with silica, after having beenexposed for 2 minutes under a Kodak Photographic Step Wedge No. 2 havinga density increment of 0.15 per step, is developed with an aqueousalkaline solution and inked up with greasy, black printing ink. Sevengradation steps can be seen. The continuous tone reproduction appearsvery smooth, 100,000 continuous tone prints of equal quality have beenproduced with such a printing plate.

An exposure time of 1 minute is required for a similar plate without thepigment coating. Only three steps are reproduced.

Example 12 A polyester film, for thin-film chromatography, having acoating of colorless silica gel, is placed coating to coating in contactwith a commercial positive working offset printing plate material of thekind used in Example 1, and a continuous tone original is placedthereon. After exposure for 5 minutes under an arc lamp in a vacuumframe, and after developing and inking up with greasy, black printingink, the continuous tones are readily visible. Naturally, thereproduction is somewhat less sharp, however.

A corresponding printing plate material which was exposed only under thecontinuous tone original required only 1 minute of exposure time, butreproduced the continuous tones poorly.

Example 13 Commercial positive working offset printing plate materialsof the kind used in Example 1 were vapor-treated with silver at an angleof about 45 in a vacuum (about 0.0001 mm. Hg), to give variousthicknesses of coating. When the silver coatings are too dense the platecannot later be correctly developed. If the silver coatings are toothin, the desired effect is only faintly discernible. With a mediumdensity of the silver, and after exposure for 4 minutes under an arclamp beneath an unscreened contiuous tone original, after developing andinking up with greasy, black printing ink, continuous tones such asclouds are reproduced true to the original. The pictures are verysmooth.

A non-vapor-treated, but otherwise similar printing plate materialrequires an exposure time of 1 minute under otherwise similarconditions. Continuous tones such as clouds are reproduced in a poormanner.

When the printing plate materials are treated vertically With metalvapor, the reproduction of the continuous tones is somewhat poorer thanin the case of oblique treatment.

To facilitate the developing of metal-vaporized printing platematerials, it is advantageous to provide the plate materials beforehandwith a water-soluble coating such as, for example, a 0.5% aqueoussolution of poly-N-vinyl N-methyl-acetamide (K=88.1).

Example '14 Polyester films, the surface of which has been renderedmatte to a roughened depth of up to 5 microns are treated With silvervapor in a vacuum and at an angle of about 45 to give variousthicknesses of coating.

A polyester film metallized in this way is placed with the metal coatingon the copying coating of a commercial positive working offset printingplate material, and a Kodak Photographic Step Wedge No. 2 is then placedthereon and finally exposure is carried out in a vacuum frame under anarc lamp. The best results were obtained with a metallized polyesterfilm requiring an exposure time of 9 minutes. After developing andinking up the resulting printing plate, nine gradation steps could beobserved. Without the application of the metallized polyester film onlyone minute of exposure time is required. Only three tone-graded stepsare reproduced, however.

Similar results are obtained with vertically metallized matte films.

Example 15 A commercial positive working offset printing plate materialof the kind used in Example 1 is coated in the manner described inExample 5 with a metallic grain suspension prepared as follows:

8.2 g. of silver nitrate are dissolved in 500 ml. of water, and 250 ml.of concentrated ammonia and 15 drops of 2-N caustic soda are addedthereto. The resulting solution is stirred into a solution of 4.5 ml. offormaldehyde (30%) in 200 ml. of water. The silver, deposited in finegrains, is washed with water and ground for 24 hours in a ball mill in50 ml. of a 10% aqueous solution of poly-N- vinyl-N-methyl acetamide(K=88.1) as a binding agent and 0.5 ml. of Arcopal N as a wetting agent.In the dried coating, the deposited silver particles are mostly up to 5microns in size and partly up to 10 microns.

The printing plate material coated in this manner is exposed for twominutes under an arc lamp beneath a Kodak Photographic Step Wedge No. 2,with a density increment of 0.15 per step. After developing with anaqueous alkaline developer and inking up with greasy, black printingink, eight gradation steps can be recog nized. The picture appears verysmooth.

Without the coating of metal grains, an exposure time of 1 minute isrequired for the same printing plate material and the developed andinked up printing plate gives only about three such steps.

Example 16 A transparent cellulose acetate film is coated with thefine-grained metal dispersion of Example 15.

The coated cellulose acetate film is placed with the coating on thecopying coating of a comercial positive working offset printing platematerial of the kind used in Example 1 and a continuous tone original islaid thereon. Exposure takes place in a vacuum frame under an arc lamp.The exposure times and the continuous tone reproduction are similar tothose given in Example 15.

Example 17 An acetate film is coated by wiping it over with a solutionof poly-N-vinyl-methyl acetamide in water and then evenly dusted,through a sieve having a mesh having a mesh of about 30 microns, withcolorless glass beads which are between 5 and 30 microns in size, sothat a densely packed layer of glass beads adheres to the tacky acetatefilm. After drying, a commercial positive working printing platematerial of the kind used in Example l is exposed with a continuous toneoriginal laid directly on the plate and with the above-described filmplaced on top. After an exposure time of 2 minutes under an 18 amp arclamp at a distance of 60 cm. in a vacuum frame, and after developing for1 minute with an aqueous alkaline developer and inking up with greasy,black printting ink, a printing plate is obtained which equals theoriginal as regards tone values and resolution capacity.

Example 18 A commercial positive working offset printing plate materialof the kind used in Example 1 is thinly smeared with with a 5% aqueoussolution of poly-N-vinyl-methyl-acetamide and is evenly dusted, througha sieve having a mesh of about 30 microns, with colorless glas beads asin Example 17. After drying, exposure takes place for 2 minutes in avacuum frame under an 18-amp arm lamp at a dis-- tance of 60 cm. througha continuous tone original. After rinsing off the glass bead layer withwater, developing for 1 minute with an aqueous alkaline developer andinking up with greasy printing ink, a printing plate is obtained whichequals the original as regards tone values and resolution capacity.

Example 19 2 g. of colorless glass beads having a particle size of 5 to30 microns are suspended in ml. of glycol monomethyl ether. 40 ml. of anacetate lacquer solution, consisting of about 13% of cellulosetriacetate and about 87% of a solvent consisting of 97.5 parts by weightof methylene chloride and 2.5 parts by Weight of isopropanol, are addedthereto and dispersed evenly by stirring and a film is cast on to aglass plate with the help of a doctor (1 mm. gap). After the film hasdried, exposure takes place for 2 minutes in a vacuum frame under an 18amp arc lamp at a distance of 60 cm, the film, as described above, beinglaid on a. commercial positive working offset printing plate material ofthe kind used in Example 1, and a continuous tone original being laid ontop. After developing for 1 minute with an aqueous alkalinesolution andthen inking up with greasy, black printing ink, a printing plate isobtained which equals the original as regards tone values and resolutioncapacity.

1 2 Example 20 In the manner described in Example 5 a covering layer ofHansa Yellow 10 G is applied to a completely smooth aluminum foilsupport, which, to reinforce the mechanical strength is backed withstill paper, and which is provided with a light-sensitive layer whichcontains as a light-sensitive substance an o-naphthoquinone diazidesulfonic acid ester produced in accordance with British PatentSpecification No. 1,039,475.

The printing plate material coated in this manner is exposed for 1.5minutes under an arc lamp beneath a Kodak Photographic Step Wedge No. 2having a density increment of 0.15 per step. After washing 01f thepigment layer with water, immersing for 2 minutes in an alkalineaqueousdeveloper and inking up with greasy, black printing ink, six gradationsteps can be observed.

A printing plate material without the pigment coating, but otherwisesimilar, is fully exposed in about a half-minute but reproduces only twoto at most three such steps.

Example 21 To a plate of electrolytically roughened aluminum having anoxide coating 2 to 3 microns thick, there is applied a negative workinglight-sensitive coating in accordance with German Patent SpecificationNo. 1,106,181, Example 1, which coating consists of epoxide resins,which are condensed with tartaric acid and esterified with cinnamicacid, and also contains Michlers ketone. A coating of finely dividedsilica, as in Example 11, is applied to this coating. After exposureunder an arc lamp for 6 minutes under a Kodak Photographic Step WedgeNo. 2 having a density increment of 0.15 per step, followed by immersionfor 1 minute in a dioxane developer and wiping with an alkaline-aqueousdeveloper, five gradation steps can be seen after inking up with greasy,black printing ink.

A photo-polymerizable printing plate material, without the coveringlayer of silica, but otherwise produced and treated in exactly the sameway, is exposed in 2 minutes but shows only two steps after developing.

Example 22 To a plate of brushed aluminum, in the manner specified inExample 1 of German Patent Specification No. 838,699, is applied anegative working light-sensitive coating which, in addition to bindingagents such as poly-N- vinylpyrrolidone and dyes such as eosin, alsocontains as a light-sensitive compound4,4-diazido-stilbene-2,2'-disulfonic acid sodium salt. A KodakPhotographic Step Wedge No. 2 having a density increment of 0.15 perstep is laid on this coating in the place of an original and on thisthere is placed a cellulose acetate film having a coating of finelydivided silica as in Example 11. After exposure for 2 minutes under anarc lamp and immersing in methanol for 1 minute for development,followed by inking up with a 0.5% methyl violet solution in equal partsof water and methanol, ten gradation steps can be observed.

An exactly similar offset printing plate material, apart from thepresence of the covering layer of silica, requires only 1 minuteexposure time, but shows only about tfive steps after inking up.

Example 23 An etchable light-sensitive printing plate material having azinc support, made in accordance with German Patent Specification No.1,195,166, with a copying layer containing as a light-sensitivesubstance, an o-naphthoquinone diazide sulfonic acid ester and aboutthree times as much novolak resin, is exposed for 3 minutes under an arclamp and under a Kodak Phographic Step Wedge No. 2 having a densityincrement of 0.15 per step, and under a film dusted with fine glassbeads as in Example 19. After developing for 3 minutes in anaqueous-alkaline developer, to which about 15% by volume of methylglycol is added, the plate is dried and is etched for 6 minutes 13 bythe conventional single-stage method at 27 C. with dilute nitric acid,using an edge protecting agent. In this way one step is etched outhaving a fine grain.

When the same etchable light-sensitive printing plate material wasprovided with a coating of finely divided silica as in Example 11, theexposure time was 4 minutes and, upon etching, one fine-grained step wasobtained.

With the same light sensitive printing plate material, only 2 minutesare required for exposure beneath a photographic step wedge Withoutusing a coating providing a shadow pattern, and under the same etchingconditions as above no step is etched out. I

Example 24 A commercial positive working otfset printing plate materialof the kind used in Example 1 is exposed for 1 minute under an arc lampin a vacuum frame under a Kodak Photographic Step Wedge No. 2 having adensity increment of 0.15 per step and under a polyamide cloth ofcolorless filaments, about microns thick, having a 10 micron mesh. Aftertreatment in an alkaline developer and inking up with greasy, blackprinting ink, six gradation steps can be seen.

A similar printing plate material, minus the polyamide cloth, butotherwise similarly treated, required an exposure time of 30 sec. andreproduced only four steps.

Example 25 A commercial positive working offset printing plate materialof the kind used in Example 1 is exposed in a vacuum frame for 2.5minutes under an arc lamp, under a Kodak Photographic Step Wedge No. 2having a density increment of 0.15 per step, and also under a finecopper mesh. The copper mesh consists of a copper foil, about 8 micronsthick, having a mesh width of about 30 microns and webs about 7 micronswide. The copper mesh becomes strongly heated during the exposure andtransmits the heat to the printing plate, this being thereby damaged. Toobviate this elfect, a filter glass (KG 3), suitable for absorbingthermal radiation, and a filter glass (BG 12) suitable for absorbing thenon-actinically effective visible radiation were interposed, both being2 mm. thick and being produced by the Jenaer Glaswerke Schott und Gen.,Mainz, Germany. After alkaline development and inking up with greasy,black printing ink, five to six gradation steps can be observed.

When the copper mesh and filters are not used, only 30 seconds arerequired for the exposure, but only four steps are reproduced.

Example 26 A commercial positive working offset printing plate materialof the kind used in Example 1 is exposed for 20 seconds under an arclamp and under an acetate film coated with finely divided silica as inExample 11. A sample, taken from the printing plate material after thisexposure treatment, is subjected for 2 minutes to alkaline developmentand, after inking up with greasy, black printing ink, exhibitssubstantially the full tone. The printing plate material, pre-exposed inthis way, after any length of storage time, is further exposed for 1minute beneath a Kodak Photographic Step Wedge No. 2 having a densityincrement of 0.15 per step. After alkaline development and inking up'With greasy, black printing ink, seven to eight gradation steps can beobserved.

Corresponding results with similar exposure times are obtained in thecase of pre-exposure beneath a film with glass beads embedded therein,the film being produced as in Example 19.

Without pre-exposure, i.e. for the exposure of the commercial materialbeneath a Kodak Photographic Step Wedge only, a period of 1 minute isrequired for corresponding full exposure. After alkaline development andinking up with greasy, black printing ink, three to four gradation stepscan be seen.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A process for the production of a printing plate capable ofreproducing continuous tones which comprises exposing under a continuoustone original a light-sensitive layer on a base suitable for theproduction of a printing plate, the light-sensitive layer being coatedwith a contrast layer containing particles of a pigment of a diameter inthe range of 0.05 to 10 microns,

the particles being distributed in the contrast layer so as to cover 10to percent of the light-sensitive layer and the exposure light beingunscrecned and traversing the contrast layer before it traverses thelightsensitive layer,

and developing the resulting image.

2. A process according to claim 1 in which the contrast layer comprisesa binding agent soluble in an alkaline solution.

3. A process according to claim 1 in which the lightsensitive layercomprises a sensitizer selected from the group consisting of aromaticdiazo and azido compounds, and photopolymerizable compounds.

4. A process according to claim 2 in which the particles of the contrastlayer are yellow pigments.

References Cited UNITED STATES PATENTS 1,161,824 11/1915 Morris 961161,849,036 3/ 1932 Ernst 961 16 2,150,805 3/ 1939 Meulendyke 961163,130,669 4/1964 Cooke 96-116 3,148,064 9/ 1964 Rauner et al. 96363,530,779 9/ 1970 Alofs 9681 DAVID KLEIN, Primary Examiner US. Cl. X.R.9638, 45

UNITED STATES. PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,829,315 Dat d August 13,1974

lnven ofl Gunther Schadlich, Renate I-Iaenisoh, and Roland Moraw It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1 after line 10, the following should be inserted:

- Claims priority, application Germanyv May 26, 1967,

K 62 378 D(a/57d Column 3, line 68, "irradiation" should read irradiatedColumn 4, line 11, "screen" should read screening. Line 14 "tond" shouldread and Line 34, "degrees" should read -degree---.

Column 11, line 47, "glas" should read glass Signed and sealed this 17thday of December 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. 7 C. MARSHALL DANN Attesting Officer Commissioner"of Patents FORM po'wso USCOMM-DC wan-Poo U.S. GOVERNMENT PRINTINGOFFICE "C9 0-36-334.

